Seismic risk for a structure depends on several interrelated factors. Of fundamental importance is the proximity of faults capable of sustaining a major slip. Other important factors include the depth range of the dislocation, the strength of the rocks being fractured, the rupture velocity, and the local geologic conditions near the structure. In order to assess the impact of nearby earthquakes on man-made structures, we have undertaken a study to design a simple computer model that simulates strong motion records near strike-slip earthquakes. Because of the engineering objectives for these simulations, the study is focused primarily on modeling response spectra at frequencies from 1 to 20 Hz. The computer model has been tested by predicting response spectra for the 1940 Imperial Valley, 1966 Parkfield, and 1976 Brawley Earthquakes.
The traditional method for assigning seismic risk is based on averaging and scaling response spectra from past earthquakes. Such scaling is based-on the magnitude of the earthquake and the distance from the fault to a receiver or site. We have extended this procedure by using a simple model of earthquake rupture in an idealized earth to predict ground motion. This model includes parameters describing fault geometry, slip characteristics and geologic structure. Field observations, laboratory measurements, and recordings from past earthquakes are used to establish values for the parameters. The procedure makes use of several lengthy computer codes to implement the earthquake model. The combined computer procedures then serve to predict the motion for a specific location from a hypothetical rupture.
The elements of the computer model can be visualized from Figure 1. The model answers the following questions: How does rupture propagate across the fault surface? What is the nature of slip at each point on the fault surface? And, how does the seismic energy propagate from the fault surface to the receiver?
The following section describes the computer model in more detail and the final section describes the results of validation studies.
A three-dimensional finite element code (SWIS) was used in conjunction with analytical solutions and laboratory experiments to provide information on how fault slip occurs during an earthquake (Archuleta and Frazier, 1978). Experiments performed on compressed rock specimens indicate that shear fracture occurs when the shear stress exceeds some limiting value in the neighborhood of one kbar. (The actual failure strength depends on rock composition, loading range, the presence of cracks, confining pressure, and interspersed fluid.) The fracture strength of rocks can then be related to the maximum velocity of particles on the fault surface near the crack tip.
The physics of spontaneous shear fracture is contained in our characterization of fault slip. The following parameters are pertinent.